Process to Improve The Efficiency of a Membrane Filter Activated Sludge System

ABSTRACT

A process for treating BOD, nitrogen and phosphorus containing wastewater, wherein the process includes the steps of providing wastewater influent into an anoxic zone having activated sludge and mixing the wastewater influent with the activated sludge in the anoxic zone to form a mixed liquor. The process further includes providing the mixed liquor into one more further treatment zones and transferring a portion of the mixed liquor from the further treatment zone to a membrane filter wherein a filtrate is separated from sludge. The process also includes selectively recycling at least a portion of the sludge to the anoxic zone as recycled activated sludge.

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/279,354 entitled PROCESS TO IMPROVE THE EFFICIENCY OF AMEMBRANE FILTER ACTIVATED SLUDGE SYSTEM, filed on Apr. 11, 2006, whichis hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to treatment of wastewaters containing BOD,phosphorus and nitrogen, such as municipal sewage, industrialwastewaters and the like by an activated sludge process. Moreparticularly, the invention relates to a process whereby a wastewaterinfluent is mixed with a denitrified-mixed liquor under anaerobicconditions before being mixed with recycled activated sludge insubsequent aeration zones.

BACKGROUND OF THE INVENTION

Activated sludge processes have been used to remove biological oxygendemand (BOD) from municipal sewage, industrial wastewaters and the like.In such a process, a wastewater influent is mixed with amicroorganism-containing recycled biomass or activated sludge in aninitial contact zone to form a mixed liquor. At some point in theprocess, the mixed liquor is aerated with sufficient oxygen to grow andmaintain a satisfactory population of microorganisms which sorb,assimilate and metabolize the BOD of the wastewater.

In the activated sludge process disclosed in U.S. Pat. No. 3,964,998,wastewater and recycled activated sludge are mixed with mechanicalstirrers in a first stage which is operated under anoxic conditions. Themixed liquor is subsequently aerated in a second stage, subjected toanoxic conditions in a third stage, aerated in a fourth stage and thenclarified to separate an activated sludge.

In another process, wastewater and recycled activated sludge are mixedand circulated around a plurality of concentric, annular basins orchannels by a plurality of surface aeration discs or other mechanicalsurface aeration devices which churn oxygen into the upper surface ofthe mixed liquor and provide sufficient agitation to prevent settling.The mixed liquor flows from one channel to the next and finally isintroduced into a clarifier to separate an activated sludge. Thechannels can be operated as a series of complete mix reactors so thatthe dissolved oxygen content in the first channel in which thewastewater and recycled activated sludge is initially mixed is aboutzero or less and the dissolved oxygen content is subsequently increasedas the mixed liquor moves from one channel to the next.

SUMMARY OF THE INVENTION

The invention provides a process for treating BOD, nitrogen andphosphorus containing wastewater. The process comprises introducingwastewater influent into an anoxic zone having activated sludge andmixing the wastewater influent with the activated sludge in the anoxiczone to form a mixed liquor. The mixed liquor is flowed from the anoxiczone into one or more further treatments zones and from the treatmentzone to a membrane filter wherein a filtrate is separated from sludgeand recycling at least a portion of the separated sludge selectively tothe anoxic zone and/or the further treatment zone as recycled activatedsludge.

Preferably, the process may use multiple reactors in series, with theability to maintain different food to micro-organism ratios anddifferent dissolved oxygen concentrations in each reactor. Forpreference the anoxic zone may include a mixing device to assist withformation of the mixed liquor. Preferably, the anoxic zone promotessimultaneous nitrification and denitrification. For preference, themethod includes recycling a portion of mixed liquor from the furthertreatment zone to the aerobic zone for mixing therein with thewastewater and the recycled activated sludge.

In another aspect, the process comprises introducing wastewater influentinto an anaerobic zone having activated sludge and mixing the wastewaterinfluent with the activated sludge in the anaerobic zone to form a mixedliquor. The mixed liquor is introduced into an oxygen-deficit aerationzone, and denitrified mixed liquor from the oxygen-deficit aeration zoneis recycled to the anaerobic zone for mixing therein with wastewater.The process further comprises transferring the mixed liquor from theoxygen-deficit aeration zone to an oxygen-surplus aeration zone,transferring a portion of the mixed liquor from the oxygen-surplusaeration zone to a membrane filter wherein a filtrate is separated fromsludge and recycling at least a portion of the separated sludge to theoxygen-deficit aeration zone as recycled activated sludge. A secondanoxic reactor, with or without addition of a carbon source to enhancedenitrification, may be inserted between the oxygen-surplus aerationzone and the membrane filter for removal of nitrates from thewastewater.

In another aspect of the invention, a process for treatingBOD-containing wastewater comprises providing wastewater influent intoan anaerobic zone having activated sludge and mixing the wastewaterinfluent with the activated sludge in the anaerobic zone to form a mixedliquor. The mixed liquor is provided into a first aeration zonemaintained under conditions which produce a complete mix reaction andprovide insufficient oxygen to meet, the biological oxygen demand of theresulting mixed liquor. Denitrified-mixed liquor from the first aerationzone is recycled to the anaerobic zone for mixing therein withwastewater. The process further comprises transferring the mixed liquorfrom the first aeration zone to a subsequent aeration zone maintainedunder conditions which produce a complete mix reaction and providesufficient oxygen to produce an overall dissolved content of at least0.5 mg/L, transferring the mixed liquor from the subsequent aerationzone to a membrane filter wherein a filtrate is separated from sludgeand recycling at least a portion of the separated sludge to the firstaeration zone as recycled activated sludge. A second anoxic reactor,with or without addition of a carbon source to enhance denitrification,may be inserted between the subsequent aeration zone and the membranefilter for removal of nitrates from the wastewater.”

Preferably, the activated sludge recycled to the oxygen deficit aerationzone contains dissolved oxygen which may be used to assist in theaeration process. The dissolved oxygen in recycled flow is preferably ina concentration of less than 8 mg/l and more preferably in the range ofabout 4 mg/l to 8 mg/l.

The membrane filter may include one or more membrane modules eachcomprising a plurality of porous membranes, said membranes beingarranged in close proximity to one another and mounted to preventexcessive movement therebetween, and means for providing, from withinthe module, by means other than gas passing through the pores of saidmembranes, gas bubbles entrained in a liquid flow such that, in use,said liquid and bubbles entrained therein move past the surfaces of saidmembranes to dislodge fouling materials therefrom, said gas bubblesbeing entrained in said liquid by flowing said liquid past a source ofgas to draw the gas into said liquid flow.

The membrane modules may also comprise a plurality of porous hollowfibre membranes, said fibre membranes being arranged in close proximityto one another and mounted to prevent excessive movement therebetween,the fibre membranes being fixed at each end in a header, one headerhaving one or more of holes formed therein through which gas/liquid flowis introduced, and partition means extending at least part way betweensaid headers to partition said membrane fibres into groups.

Preferably, the partition means are formed by a spacing betweenrespective fibre groups. The partitions may be parallel to each otheror, in the case of cylindrical arrays of fibre membranes, the partitionsmay extend radially from the centre of the array or be positionedconcentrically within the cylindrical array. In an alternative form, thefibre bundle may be provided with a central longitudinal passageextending the length of the bundle between the headers.

The membrane module may comprise a plurality of porous hollow membranefibres extending longitudinally between and mounted at each end to arespective potting head, said membrane fibres being arranged in closeproximity to one another and mounted to prevent excessive movementtherebetween, said fibres being partitioned into a number of bundles atleast at or adjacent to their respective potting head so as to form aspace therebetween, one of said potting heads having an array ofaeration openings formed therein for providing gas bubbles within saidmodule such that, in use, said bubbles move past the surfaces of saidmembrane fibres to dislodge fouling materials therefrom.

The fibre bundle is protected and fibre movement is limited by a modulesupport screen which has both vertical and horizontal elementsappropriately spaced to provide unrestricted fluid and gas flow throughthe fibres and to restrict the amplitude of fibre motion reducing energyconcentration at the potted ends of the fibres.

Preferably, said aeration openings are positioned to coincide with thespaces formed between said partitioned bundles. For preference, saidopenings comprise a slot, slots or a row of holes. Preferably, the fibrebundles are located in the potting head between the slots or rows ofholes.

For further preference, the gas bubbles are entrained or mixed with aliquid flow before being fed through said holes or slots, though it willbe appreciated that gas only may be used in some configurations. Theliquid used may be the feed to the membrane module. The fibres and/orfibre bundles may cross over one another between the potting headsthough it is desirable that they do not.

Preferably, the fibres within the module have a packing density (asdefined above) of between about 5 to about 70% and, more preferably,between about 8 to about 55%.

For preference, said holes have a diameter in the range of about 1 to 40mm and more preferably in the range of about 1.5 to about 25 mm. In thecase of a slot or row of holes, the open area is chosen to be equivalentto that of the above holes.

Typically, the fibre inner diameter ranges from about 0.1 mm to about 5mm and is preferably in the range of about 0.25 mm to about 2 mm. Thefibres wall thickness is dependent on materials used and strengthrequired versus filtration efficiency. Typically wall thickness isbetween 0.05 to 2 mm and more often between 0.1 mm to 1 mm.

The membrane filter may also be in the form of a membrane bioreactorincluding a tank having means for the introduction of feed thereto,means for forming activated sludge within said tank, a membrane moduleaccording to the first aspect positioned within said tank so as to beimmersed in said sludge and said membrane module provided with means forwithdrawing filtrate from at least one end of said fibre membranes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic view of an improved wastewater treatmentsystem embodying the invention.

FIG. 2 illustrates a schematic view of an improved wastewater treatmentsystem of another embodiment of the invention.

FIG. 3 shows a schematic side elevation of one embodiment of a membranemodule and illustrates the method of cleaning in a membrane bioreactoremployed in the filtration apparatus of a preferred embodiment.

FIG. 4 shows an enlarged schematic side elevation of one form of the jettype arrangement used to form entrained gas bubbles of a membranebioreactor employed in the filtration apparatus of a preferredembodiment.

FIG. 5 a shows a schematic side elevation of a partitioned membranemodule of a membrane bioreactor employed in the filtration apparatus ofa preferred embodiment.

FIG. 5 b shows a section through the membrane bundle of FIG. 5 a.

FIG. 6 a shows a schematic side elevation of a partitioned membranemodule of a membrane bioreactor employed in the filtration apparatus ofa preferred embodiment.

FIG. 6 b shows a section through the membrane bundle of FIG. 6 a.

FIG. 7 a shows a schematic side elevation of a partitioned membranemodule of a membrane bioreactor employed in the filtration apparatus ofa preferred embodiment.

FIG. 7 b shows a section through the membrane bundle of FIG. 7 a.

FIG. 8 a shows a schematic side elevation of a partitioned membranemodule of a membrane bioreactor employed in the filtration apparatus ofa preferred embodiment.

FIG. 8 b shows a section through the membrane bundle of FIG. 8 a.

FIG. 9 shows a similar view to FIG. 8 of a membrane module;

FIG. 10 shows a similar view to FIG. 8 of a membrane module;

FIG. 11 shows a sectioned perspective pictorial view of the lower end ofanother preferred embodiment of the membrane module of a membranebioreactor employed in the filtration apparatus of a preferredembodiment;

FIG. 12 shows a sectioned perspective pictorial view of the upper end ofthe membrane module of FIG. 11.

FIG. 13 illustrates a schematic view of an improved wastewater treatmentsystem of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, this embodiment of the invention employs a completemix system for treating BOD, nitrogen and phosphorus containingwastewater. In the system, wastewater may or may not be pre-treated toremove grit, large particulate matter and other suspended solids. Thewastewater is then fed through a conduit into an anaerobic reactor orreactors 1, which preferably utilize a mechanical mixer (not shown) tokeep solids in suspension without aeration.

As shown in FIG. 1, denitrified mixed liquor from a downstream aerationzone 2 having a continuous oxygen deficit is added to the anaerobic zone1.

Typically, the denitrified mixed liquor is added at a rate from 0.5 to 2times the wastewater influent flow rate. In the anaerobic reactor zone1, micro-organisms capable of accumulating quantities of phosphorus inexcess of that required for simple cell growth and reproduction take-upand store simple carbon compounds, using energy derived from thehydrolysis and release of polyphosphates. The hydraulic residence timein the anaerobic zone 1 is typically, but not limited to, 0.5 to 2.0hours.

The combined wastewater and denitrified mixed liquor then flow to one ormore downstream aeration zones or tanks 2 having a continuous oxygendeficit. As used herein, the terms “aeration zone or tank having acontinuous oxygen deficit” and “oxygen-deficit aeration zone or tank”are synonymous and meant to refer to a zone having a continuous oxygendeficit maintained, e.g., by controlling the rate of aeration such thatthe rate of oxygen supply is less than the rate of oxygen consumption bythe micro-organisms in the tank. This results in dissolved oxygenconcentrations at or near zero throughout the basin.

Aerator devices, or a combination of aerator devices and mechanicalmixers provide oxygen and keep the mixed liquor solids in suspension,however, in the present embodiment, which employs a membrane filter 4with aeration, at least some of the oxygen is provided in the activatedsludge fed into the aeration zone 2 having a continuous oxygen deficitfrom the membrane filter 4. Nitrified mixed liquor from a downstreamaeration zone 3 having an oxygen surplus may also be added thereto.

Using oxygen supplied by the aerators and recycled activated sludge,heterotrophic organisms oxidize BOD and autotrophic organisms oxidizeammonia in the aeration tank 2 having a continuous oxygen deficit. Asthe amount of oxygen supplied is less than the demand and nitrates fromoxidation of ammonia are present, heterotrophic organisms will oxidizeBOD using nitrates as an electron acceptor and converting nitrates intonitrogen gas. The hydraulic residence time in the aeration zone 2 havinga continuous oxygen deficit is typically, but not limited to, 2 to 12hours.

The effluent from the aeration zone 2 having a continuous oxygen deficitthen flows to one or more downstream tanks 3 having a continuous oxygensurplus. Most of the BOD and ammonia in the wastewater has been oxidizedby the time the wastewater reaches the last aeration zone, so dissolvedoxygen concentrations of 1 mg/L or greater are easily maintained in thelast aeration zone. Preferably, the dissolved oxygen concentration is atleast 5 mg/L. Oxidation of BOD and ammonia occurs in the aeration tanks3 having a continuous oxygen surplus. In the aeration zone 3 ofcontinuous oxygen surplus, micro-organisms oxidize the carbon that theyhave absorbed in the anaerobic zone and absorb and store polyphosphatesas an energy source for the return trip back to the anaerobic zone 1 asreturn activated sludge. The hydraulic residence time in the aerationzone 3 having a continuous oxygen surplus is typically, but not limitedto, 2 to 12 hours.

Finally, effluent from the aeration tanks 3 having a continuous oxygensurplus flows to the membrane filter 4 where the biological solids areseparated. A portion of the effluent may be returned to the biologicalprocess leaving a filtrate with reduced levels of organic matter,phosphorus and nitrogen. This filtrate is removed and becomes theprocess effluent. Some of the separated solids are removed from thesystem (waste activated sludge), thereby removing phosphorus and organicmatter.

Referring to FIG. 2, a further embodiment of the invention is shownwhere a second anoxic reactor zone or tank 3′ is provided betweenaeration tank 3 and the membrane filter 4. This further anoxic reactorzone 3′ is used to remove nitrates from the wastewater stream and mayoptionally be provided with a carbon source to assist nitrate removal.

The Membrane Bioreactor

One of the components of the water treatment systems of preferredembodiments is a membrane bioreactor. Membrane bioreactor systemscombine biological treatment, involving bacteria, with membraneseparation to treat wastewater. Treated water is separated from thepurifying bacteria, referred to as activated sludge, by a process ofmembrane filtration. Membrane bioreactors typically employ submergedhollow fiber membrane modules incorporated in a distributed flowreactor.

Membrane processes can be used as an effective tertiary treatment ofsewage and provide quality effluent. Submerged membrane processes wherethe membrane modules are immersed in a large feed tank and filtrate iscollected through suction applied to the filtrate side of the membrane,and wherein the membrane bioreactor combines biological and physicalprocesses in one stage, are compact, efficient, economic, and versatile.

Membrane bioreactors that can be employed in the water treatment systemsof preferred embodiments include those commercially available fromUSFilter Memcor Research Pty. Ltd. and Zenon Environmental, Inc. Theseinclude systems such as USFilter's MEMCOR® MBR Membrane BioreactorSystem and Zenon's ZeeWeed® MBR Membrane Bioreactor process utilizingthe ZeeWeed® 500 and ZeeWeed® 1000 systems.

The membrane modules employed in the ZeeWeed® 500 system consist ofhundreds of membrane fibers oriented vertically between two headers. Themembrane is a reinforced fiber with a nominal pore size of 0.04 μm. Thehollow fibers are slightly longer than the distance between the top andbottom headers and this allows them to move when aerated. It is the airthat bubbles up between the fibers that scours the fibers andcontinuously removes solids from the surface of the membrane. From 1 to36 membrane modules are arranged in a cassette. ZeeWeed® 500 systemtypically consists of two or more parallel trains. Each train consistsof a process pump, automatic valves, and instrumentation. The followingcomponents are generally required in a system and can either bededicated to a specific train or shared amongst trains: a tank (intowhich cassettes are immersed); metering pumps (for chemical addition);membrane blowers (to provide air for scouring the membranes); reject orsludge pump; vacuum pump (for entrained air removal); “clean-in-place”pumps; backpulse or wash tank; and control system. Other components canbe employed, depending on the design or application: strainers (forpre-screening the feedwater); process blowers (biological treatmentsystems only); feed pumps; mixers; sludge recirculation pumps; andcassette removal hoist or other mechanism.

The ZeeWeed® 1000 ultrafilter membrane has a nominal pore size of 0.02μm and is designed to remove suspended solids, protozoa, bacteria, andviruses from water supplies. The ZeeWeed® 1000 system operates in a modesimilar to conventional media filters with direct (dead-end) filtrationfollowed by a periodic air and water backwash. A ZeeWeed® 1000 cassetteis made by stacking elements both vertically and horizontally in ablock. There are a variety of cassette configurations available rangingin size from 3 to 96 elements. A ZeeWeed® 1000 system consists of aseries of parallel trains. Each train consists of ZeeWeed® cassettes, aprocess pump, piping, instrumentation, and controls. The backpulse pump,blower and clean-in-place equipment can be shared amongst the trains.Feed enters each train from a feed channel that runs along the long sideof the train at the bottom of the tank. Reject is collected in troughsbetween cassettes and is discharged to the overflow channel that runsthe length of the tank.

The membrane bioreactor systems preferably employed in the preferredembodiments utilize an effective and efficient membrane cleaning method.Commonly used physical cleaning methods include backwash (backpulse,backflush) using a liquid permeate or a gas, membrane surface scrubbing,and scouring using a gas in the form of bubbles in a liquid. Examples ofthe second type of method are described in U.S. Pat. No. 5,192,456 toIshida et al., U.S. Pat. No. 5,248,424 to Cote et al., U.S. Pat. No.5,639,373 to Henshaw et al., U.S. Pat. No. 5,783,083 to Henshaw et al.,and U.S. Pat. No. 6,555,005 to Zha et al.

In the examples referred to above, a gas is injected, usually by apressurized blower, into a liquid system where a membrane module issubmerged to form gas bubbles. The bubbles so formed then travel upwardsto scrub the membrane surface to remove the fouling substances formed onthe membrane surface. The shear force produced largely relies on theinitial gas bubble velocity, bubble size, and the resultant of forcesapplied to the bubbles. The fluid transfer in this approach is limitedto the effectiveness of the gas lifting mechanism. To enhance thescrubbing effect, more gas has to be supplied. However, this method hasseveral disadvantages: it consumes large amounts of energy, it can formmist or froth flow reducing effective membrane filtration area, and canbe destructive to membranes. Moreover, in an environment of highconcentration of solids, the gas distribution system can graduallybecome blocked by dehydrated solids or simply be blocked when the gasflow accidentally ceases.

For most tubular membrane modules, the membranes are flexible in themiddle (longitudinal directions) of the modules but tend to be tighterand less flexible towards to both potted heads. When such modules areused in an environment containing high concentrations of suspendedsolids, solids are easily trapped within the membrane bundle, especiallyin the proximity of two potted heads. The methods to reduce theaccumulation of solids include the improvement of module configurationsand flow distribution when gas scrubbing is used to clean the membranes.

In the design of a membrane module, the packing density of the tubularmembranes in a module is one factor that is considered. The packingdensity of the fiber membranes in a membrane module as used herein isdefined as the cross-sectional potted area taken up by the fibermembranes divided by the total potted area and is normally expressed as,a percentage. From the economical viewpoint it is desirable that thepacking density be as high as possible to reduce the cost of makingmembrane modules. In practice, solid packing is reduced in a lessdensely packed membrane module. However, if the packing density is toolow, the rubbing effect between membranes could also be lessened,resulting in less efficient scrubbing/scouring of the membrane surfaces.It is, thus desirable to provide a membrane configuration that assistsremoval of accumulated solids while maximizing packing density of themembranes. The membranes can be in contact with each other (e.g., athigh packing densities), or can be closely or distantly spaced apart(e.g., at low packing densities), for example, a spacing between fiberwalls of from about 0.1 mm or less to about 10 mm or more is typicallyemployed.

A method of scrubbing a membrane surface using a liquid medium with gasbubbles entrained therein, including the steps of entraining the gasbubbles-into the liquid medium by flow of the liquid medium past asource of the gas, and flowing the gas bubbles and liquid medium alongthe membrane surface to dislodge fouling materials therefrom, can beemployed in membrane bioreactors.

Preferably, the gas bubbles are entrained into the liquid stream bymeans of a venturi device or other type of junction. For furtherpreference, the gas bubbles are entrained or injected into the liquidstream by means of devices which forcibly mix gas into a liquid flow toproduce a mixture of liquid and bubbles, such devices including a jet,nozzle, ejector, eductor, injector or the like. Optionally, anadditional source of bubbles can be provided in the liquid medium bymeans of a blower or like device. The gas used can include air, oxygen,gaseous chlorine, or ozone. Air is the most economical for the purposesof scrubbing and/or aeration. Gaseous chlorine can be used forscrubbing, disinfection, and enhancing the cleaning efficiency bychemical reaction at the membrane surface. The use of ozone, besides thesimilar effects mentioned for gaseous chlorine, has additional features,such as oxidizing DBP precursors and converting non-biodegradable NOM'sto biodegradable dissolved organic carbon.

The membrane modules employed in the membrane bioreactor preferablycomprise a plurality of porous membranes arranged in close proximity toone another, optionally mounted to prevent excessive movementtherebetween, and include a source of gas bubbles for providing, fromwithin the module gas bubbles entrained in a liquid flow such that, inuse, the liquid and bubbles entrained therein move past the surfaces ofthe membranes to dislodge fouling materials therefrom, the gas bubblesbeing entrained in the liquid by flowing the liquid past a source of gasto draw the gas into the liquid flow. Preferably, the liquid and bubblesare mixed and then flowed past membranes to dislodge the foulingmaterials.

The fibers of the membrane bioreactor can be cleaned by providing, fromwithin the array of fibers, by means other than gas passing through thepores of the membranes, uniformly distributed gas-bubbles entrained in aliquid flow, the gas bubbles being entrained in the liquid flow byflowing the liquid past a source of gas so as to cause the gas to bedrawn and/or mixed into the liquid, the distribution being such that thebubbles pass substantially uniformly between each membrane in the arrayto, in combination with the liquid flow, scour the surface of themembranes and remove accumulated solids from within the membrane module.Preferably, the bubbles are injected and mixed into the liquid flow.

Preferably, the membranes of the membrane bioreactor comprise poroushollow fibers, the fibers being fixed at each end in a header, the lowerheader having one or more holes formed therein through which gas liquidflow is introduced. The holes can be circular, elliptical or in the formof a slot. The fibers are normally sealed at the lower end and open attheir upper end to allow removal of filtrate, however, in somearrangements, the fibers can be open at both ends to allow removal offiltrate from one or both ends. The fibers are preferably arranged incylindrical arrays or bundles, however other configurations can also beemployed, e.g., square, hexagonal, triangular, irregular, and the like.It will be appreciated that the cleaning process described is equallyapplicable to other forms of membrane such flat or plate membranes thatcan also be employed in membrane bioreactors.

The membrane modules employed in the membrane bioreactor preferablycomprise a plurality of porous hollow fiber membranes, the fibermembranes being arranged in close proximity to one another and mountedto prevent excessive movement therebetween, the fiber membranes beingfixed at each end in a header, one header having one or more of holesformed therein through which gas/liquid flow is introduced, andpartition means extending at least part way between the headers topartition the membrane fibers into groups. Preferably, the partition orpartitions are formed by a spacing between respective fiber groups,however porous (e.g., a screen, clip, or ring) or solid partitions canalso be employed. The partitions can be parallel to each other or, inthe case of cylindrical arrays of fiber membranes, the partitions canextend radially from the center of the array or be positionedconcentrically within the cylindrical array. In an alternative form, thefiber bundle can be provided with a central longitudinal passageextending the length of the bundle between the headers.

The membrane modules employed in a membrane bioreactor preferablyinclude a plurality of porous hollow membrane fibers extendinglongitudinally between and mounted at each end to a respective pottinghead, the membrane fibers being arranged in close proximity to oneanother and mounted to prevent excessive movement therebetween, thefibers being partitioned into a number of bundles at least at oradjacent to their respective potting head so as to form a spacetherebetween, one of the potting heads having an array of aerationopenings formed therein for providing gas bubbles within the module suchthat, in use, the bubbles move past the surfaces of the membrane fibersto dislodge fouling materials therefrom.

The fiber bundle can be protected and fiber movement can be limited by amodule support screen which has both vertical and horizontal elementsappropriately spaced to provide unrestricted fluid and gas flow throughthe fibers and to restrict the amplitude of fiber motion reducing energyconcentration at the potted ends of the fibers. Alternatively, clips orrings can also be employed to bind the fiber bundle.

Preferably, the aeration openings are positioned to coincide with thespaces formed between the partitioned bundles. Preferably, the openingscomprise one or more holes or slots, which can be arranged in variousconfigurations, e.g., a row of holes. Preferably, the fiber bundles arelocated in the potting head between the slots or rows of holes. Incertain embodiments, it can be preferred to situate the holes or slotswithin the fiber bundles, or both within and adjacent to the fiberbundles.

Preferably, the gas bubbles are entrained or mixed with a liquid flowbefore being fed through the holes or slots, though it will beappreciated that gas only can be used in some configurations. The liquidused can be the feed to the membrane module. The fibers and/or fiberbundles can cross over one another between the potting heads though itis desirable that they do not.

Typically, the fibers within the module have a packing density (asdefined above) of from about 5% or less to about 75% or more, preferablyfrom about 6, 7, 8, 9, or 10% to about 60, 65, or 70%, and morepreferably from about 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20% toabout 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,or 55%.

Preferably, the holes have a diameter of from about 0.5 mm or less toabout 50 mm or more, more preferably from about 0.6, 0.7, 0.8, 0.9, 1.0,1.1, 1.2, 1.3, 1.4, or 1.5 to about 25, 30, 35, 40, or 45 mm, and mostpreferably from about 1.75, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 mm toabout 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,23, or 24 mm. In the case of a slot or row of holes, the open area ischosen to be equivalent to that of the above holes.

Typically, the fiber inner is from about 0.05 mm or less to about 10 mmor more, preferably from about 0.10, 0.15, or 0.20 mm to about 3, 4, 5,6, 7, 8, or 9 mm, and preferably from about 0.25, 0.50, 0.75, or 1.0 mmto about 1.25, 1.50, 1.75, 2.00, or 2.50 mm. The fibers wall thicknesscan depend on materials used and strength required versus filtrationefficiency. Typically, wall thickness is from about 0.01 mm or less toabout 3 mm or more, preferably from about 0.02, 0.03, 0.04, 0.05, 0.06,0.07, 0.08, or 0.09 mm to about 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8,1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or 2.9 mm, and mostpreferably from about 0.1, 0.2, 0.3, 0.4, or 0.5 mm to about 0.6, 0.7,0.8, 0.9, or 1 mm.

The membrane bioreactor can include a tank having a line, a pipe, apump, and or other apparatus for the introduction of feed thereto, anactivated sludge within the tank, a membrane module positioned withinthe tank so as to be immersed in the sludge, and apparatus forwithdrawing filtrate from at least one end of the fiber membranes.

The membrane bioreactor is preferably operated by introducing feed tothe tank, applying a vacuum to the fibers to withdraw filtrate therefromwhile intermittently, cyclically, or continuously supplying gas bubblesthrough the aeration openings to within the module such that, in use,the bubbles move past the surfaces of the membrane fibers to dislodgefouling materials therefrom. Preferably, the gas bubbles are entrainedor mixed with a liquid flow when fed through the holes or slots.

If desired, a further source of aeration can be provided within the tankto assist microorganism activity. Preferably, the membrane module issuspended vertically within the tank and the further source of aerationcan be provided beneath the suspended module. Alternatively, the modulecan be suspended horizontally, or in any other desired position.Preferably, the further source of aeration comprises a group of airpermeable tubes or other such aeration source. The membrane module canbe operated with or without backwash, depending on the flux. A highmixed liquor of suspended solids (about 5,000 ppm or less to about20,000 ppm or more) in the bioreactor has been shown to significantlyreduce residence time and improve filtrate quality. The combined use ofaeration for both degradation of organic substances and membranecleaning has been shown to enable constant filtrate flow withoutsignificant increases in transmembrane pressure while establishing, highconcentration of mixed liquor suspended solids (MLSS). The use ofpartitioned fiber bundles enables higher packing densities to beachieved without significantly compromising the gas scouring process.This provides for-higher filtration efficiencies to be gained.

In a particularly preferred embodiment, a module as described in U.S.Pat. No. 6,555,005 is employed in the membrane bioreactor. Referring toFIG. 3, the membrane module 5 typically comprises fiber, tubular, orflat sheet form membranes 6 pofted at two ends 7 and 8 and optionallyencased in a support structure, in this case a screen 9. Either one orboth ends of the membranes can be used for the permeate collection. Thebottom of the membrane module has a number of through apertures 10 inthe pot 11 to distribute a mixture of gas and liquid feed past themembrane surfaces. A venturi device 12 or the like is connected to thebase of the module. The venturi device 12 intakes gas through inlet 13,mixes or entrains the gas with liquid flowing through feed inlet 14,forms gas bubbles and diffuses the liquid/gas mix into the moduleapertures 10. After passing through the distribution apertures 10, theentrained gas bubbles scrub membrane surfaces while travelling upwardsalong with the liquid flow. Either the liquid feed or the gas can be acontinuous or intermittent injection depending on the systemrequirements. With a venturi device it is possible to create gas bubblesand aerate the system without a blower. The venturi device 12 can be aventuri tube, jet, nozzle, ejector, eductor, injector, or the like.

Referring to FIG. 4, an enlarged view of jet or nozzle type device 15 isshown. In this embodiment, liquid is forced through a jet 16 having asurrounding air passage 17 to produce a gas entrained liquid flow 18.Such a device allows the independent control of gas and liquid medium-byadjusting respective supply valves.

The liquid commonly used to entrain the gas is the feed water,wastewater, or mixed liquor to be filtered. Pumping such an operatingliquid through a venturi or the like creates a vacuum to suck the gasinto the liquid, or reduces the gas discharge pressure when a blower isused. By providing the gas in a flow of the liquid, the possibility ofblockage of the distribution apertures 10 is substantially reduced.

By using a venturi device or the like it is possible to generate gasbubbles to scrub membrane surfaces without the need for a pressurizedgas supply such as a blower. When a motive fluid passes through aventuri it generates a vacuum to draw the gas into the liquid flow andgenerate gas bubbles therein. Even if a blower is still required, theuse of the above process reduces the discharge pressure of the blowerand therefore lowers the cost of operation. The liquid and gas phasesare well mixed in the venturi and then diffuse into the membrane moduleto scrub the membranes. Where a jet type device is used to forcibly mixthe gas into the liquid medium, an additional advantage is provided inthat a higher velocity of bubble stream is produced. In treatment ofwastewater, such thorough mixing provides excellent oxygen transfer whenthe gas used is air or oxygen. If the gas is directly injected into apipe filled with a liquid, it is impossible that the gas will form astagnant gas layer on the pipe wall and therefore gas and liquid willbypass into different parts of a module, resulting in poor cleaningefficiency. The flow of gas bubbles is enhanced by the liquid flow alongthe membrane resulting in a large scrubbing shear force being generated.This method of delivery of gas/liquid provides a positive fluid transferand aeration with the ability to independently adjust flow rates of gasand liquid. The injection of a mixture of two-phase fluid (gas/liquid)into the holes of the air distribution device can eliminate theformation of dehydrated solids and therefore prevent the gradualblockage of the holes by such dehydrated solids. The injectionarrangement further provides an efficient cleaning mechanism forintroducing cleaning chemicals effectively into the depths of the modulewhile providing scouring energy to enhance chemical cleaning. Thisarrangement, in combination with the high packing density obtainablewith the module configuration described, enables the fibers to beeffectively cleaned with a minimal amount of chemicals. The moduleconfiguration described allows a higher fiber packing density in amodule without significantly increasing solid packing. This adds anadditional flexibility that the membrane modules can be eitherintegrated into the aerobic basin or arranged in a separate tank. In thelatter arrangement, the advantage is a significant saving on chemicalusage due to the small chemical holding in the tank and in labor costsbecause the chemical cleaning process can be automated. The reduction inchemicals used is also important because the chemicals, which can be fedback to the bio process, are still aggressive oxidizers and thereforecan have a deleterious effect on bio process. Accordingly, any reductionin the chemical load present in the bio-process provides significantadvantages.

The positive injection of a mixture of gas and liquid feed to eachmembrane module provides a uniform distribution of process fluid aroundmembranes and therefore minimizes the feed concentration polarizationduring filtration. The concentration polarization is greater in alarge-scale system and for the process feed containing large amounts ofsuspended solids. The prior art systems have poor uniformity because theprocess fluid often enters one end of the tank and concentrates as itmoves across the modules. The result is that some modules must deal withmuch higher concentrations than others, resulting in inefficientoperation. The filtration efficiency is enhanced due to a reducedfiltration resistance. The feed side resistance is decreased due to areduced transverse flow passage to the membrane surfaces and theturbulence generated by the gas bubbles and the two-phase flow. Such acleaning method can be used to the treatment of drinking water,wastewater, and the related processes by membranes. The filtrationprocess can be driven by suction or pressurization.

Referring to FIGS. 5 to 6, embodiments of various partitioningarrangements are shown. Again these embodiments are illustrated withrespect to cylindrical tubular or fiber membrane bundles 20, however, itwill be appreciated that other configurations can be employed. FIGS. 5 aand 5 b show a bundle of tubular membranes 20 partitioned verticallyinto several thin slices 21 by a number of parallel partition spaces 22.This partitioning of the bundle enables accumulated solids to be removedmore easily without significant loss of packing density. Suchpartitioning can be achieved during the potting process to form completepartitions or partial partitions. Another method of forming apartitioned module is to pot several small tubular membrane bundles 23into each module as shown in FIGS. 5 a and 5 b.

Another configuration of membrane module is illustrated in FIGS. 7 a and7 b. The central membrane-free zone forms a passage 24 to allow for moreair and liquid injection. The gas bubbles and liquid then travel alongthe tubular membranes 20 and pass out through arrays of fibers at thetop potted head 8, scouring and removing solids from membrane walls. Asingle gas or a mixture of gas/liquid can be injected into the module.

FIGS. 8 a and 8 b illustrate yet a further embodiment similar to FIG. 4but with single central hole 30 in the lower pot 7 for admission of thecleaning liquid/gas mixture to the fiber membranes 20. In thisembodiment, the fibers are spread adjacent the hole 30 and converge indiscrete bundles 23 toward the top pot 8. The large central hole 30 hasbeen found to provide greater liquid flow around the fibers and thusimproved cleaning efficiency.

FIGS. 9 and 10 show further embodiments of the invention having asimilar membrane configuration to that of FIGS. 8 a and 8 b and jetmixing system similar to that of the embodiment of FIG. 4. The use of asingle central hole 30 allows filtrate to drawn off from the fibers 20at both ends as shown in FIG. 10.

Referring to FIGS. 11 and 12 of the drawings, the module 45 comprises aplurality of hollow fiber membrane bundles 46 mounted in and extendingbetween an upper 47 and lower potting head 8. The potting heads 47 and48 are mounted in respective potting sleeves 49 and 50 for attachment toappropriate manifolding (not shown). The fiber bundles 46 are surroundedby a screen 51 to prevent excessive movement between the fibers.

As shown in FIG. 11, the lower potting head 48 is provided with a numberof parallel arranged slot type aeration holes 52. The fiber membranes 53are potted in bundles 46 to form a partitioned arrangement having spaces54 extending transverse of the fiber bundles. The aeration holes 52 arepositioned to generally coincide with the partition spaces, though thereis generally a number of aeration holes associated with each space.

The lower potting sleeve 50 forms a cavity 55 below the lower pot 48. Agas or a mixture of liquid and gas is injected into this cavity 55, by ajet assembly 57 (described earlier) before passing through the holes 52into the membrane array.

In use, the use of partitioning enables a high energy flow of scouringgas and liquid mixture, particularly near the pot ends of the fiberbundles, which assist with removal of buildup of accumulated solidsaround the membrane fibers.

Air is preferably introduced into the module continuously to provideoxygen for microorganism activities and to continuously scour themembranes. Alternatively, in some embodiments, pure oxygen or other gasmixtures can be used instead of air. The clean filtrate is drawn out ofthe membranes by a suction pump attached to the membrane lumens thatpass through the upper pot, or the filtrate can be drawn out of themembranes from the lower pot by gravity or suction pump.

Preferably, the membrane module is operated under low transmembranepressure (TMP) conditions due to the high concentration of mixed liquorsuspended solids (MLSS) present in the reactor. Higher transmembranepressure can advantageously be employed for reduced concentrations ofsuspended solids.

It has been found that the module system of preferred embodiments ismore tolerant of high MLSS than many other systems and the efficient airscrub and back wash (when used) assists efficient operation andperformance of the bioreactor module.

Any suitable membrane bioreactor can be employed in the water treatmentsystems of the preferred embodiments. A particularly preferred membranebioreactor system is designed to draw filtrate from a reservoir ofliquid substrate by the use of vertically oriented microporous hollowfibers immersed within the substrate.

Any suitable substrate can be filtered using the methods and apparatusof the preferred embodiments. Suitable substrates include, but are notlimited to, ground water, river water, drinking water,organic-containing substrates such as sewage, agricultural run-off,industrial process water, and the like. While water-containingsubstrates are particularly amenable to the methods and apparatusdescribed herein, substrates containing other liquids can also befiltered (e.g., ethanol, or other chemicals).

The membrane bioreactor filtration unit includes a filtrate sub-manifoldand an air/liquid substrate sub-manifold, which receive the upper andlower ends, respectively, of four sub-modules. Each sub-manifoldincludes four circular fittings or receiving areas, each of whichreceives an end of one of the sub-modules. Each sub-module isstructurally defined by a top cylindrical pot, a bottom cylindrical pot,and a cage connected therebetween to secure the fibers. The pots securethe ends of the hollow fibers and are formed of a resinous or polymericmaterial. The ends of the cage are fixed to the outer surfaces of thepots. Each pot and associated end of the cage are together receivedwithin one of the four circular fittings of each sub-manifold. Thesub-manifolds and pots of the sub-modules are coupled together in afluid-tight relationship with the aid of circular clips and O-ringseals. The cage provides structural connection between the pots of eachsub-module.

Each sub-module includes fibers arranged vertically between its top andbottom pot. The fibers have a length somewhat longer than the distancebetween the pots, such that the fibers can move laterally. The cageclosely surrounds the fibers of the sub-module so that, in operation,the outer fibers touch the cage, and lateral movement of the fibers isrestricted by the cage. The lumens of the lower ends of the fibers aresealed within the bottom pot, while the upper ends of the fibers are notsealed. In other words, the lumens of the fibers are open to the insideof the filtrate sub-manifold above the upper face of the top pot. Thebottom pot includes a plurality of slots extending from its lower faceto its upper face, so that the mixture of air bubbles and liquidsubstrate in the air/liquid substrate sub-manifold can flow upwardthrough the bottom pot to be discharged between the lower ends of thefibers.

The filtrate sub-manifold is connected to a vertically oriented filtratewithdrawal tube that in turn connects to a filtrate manifold thatreceives filtrate from all of the filtration units of a rack. Thefiltrate withdrawal tube is in fluid communication with the upper facesof the top pots of the sub-modules, so that filtrate can be removedthrough the withdrawal tube. In addition, the system includes an airline that provides air to the air/liquid substrate sub-module skirt.

In operation, the cages of the sub-modules admit the liquid substrateinto the region of the hollow fibers, between the top and bottom pots. Apump is utilized to apply suction to the filtrate manifold and, thus,the filtrate withdrawal tubes and fiber lumens of the sub-modules. Thiscreates a pressure differential across the walls of the fibers, causingfiltrate to pass from the substrate into the lumens of the fibers. Thefiltrate flows upward through the fiber lumens into the filtratesub-manifold, through the filtrate withdrawal tube, and upward into thefiltrate manifold to be collected outside of the reservoir.

During filtration, particulate matter accumulates on the outer surfacesof the fibers. As increasing amounts of particulate matter stick to thefibers, it is necessary to increase the pressure differential across thefiber walls to generate sufficient filtrate flow. To maintaincleanliness of the outer surfaces of the fibers, air and liquidsubstrate are mixed in the skirt of the air/liquid substrate sub-moduleand the mixture is then distributed into the fiber bundles through theslots of the bottom pots and is discharged as a bubble-containingmixture from the upper faces of the bottom pots. Continuous,intermittent, or cyclic aeration can be conducted. It is particularlypreferred to conduct cyclic aeration, wherein the air on and air offtimes are of equal length, and the total cycle time (time of one air onand one air off period), is from about 1 second or less to about 15minutes or more, preferably from about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, or 14 second to about 6, 7, 8, 9, 10, 11, 12, 13, or 14 minutes,and more preferably from about 15, 20, 25, 30, 35, 40, 45, 50, 55, 60,65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, or 120 seconds to about130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260,270, 280, 290, or 300 seconds. The rising bubbles scour (i.e., cleanparticulate matter from) the fiber surfaces. Aeration wherein the air isprovided in uniform bubble sizes can be provided, or a combination ofdifferent bubble sizes can be employed, for example, coarse bubbles orfine bubbles, simultaneously or alternately. Regular or irregular cycles(in which the air on and air off times vary) can be employed, as cansinusoidal, triangular, or other types of cycles, wherein the rate ofair is not varied in a discontinuous fashion, but rather in a gradualfashion, at a preferred rate or varying rate. Different cycle parameterscan be combined and varied, as suitable.

In a particularly preferred embodiment, fine bubbles are continuouslyprovided to the membrane bioreactor for aeration, while coarse bubblesare provided cyclically for scouring. Bubbles are typically from about0.1 or less to about 50 mm or more in diameter. Bubbles from about 0.1to about 3.0 mm in diameter, preferably from about 0.2, 0.3, 0.4, 0.5,0.6, 0.7, 0.9, 0.9, or 1.0 mm to about 1.25, 1.50, 1.75, 2.00, 2.25,2.50 or 2.75 mm in diameter are particularly effective in providingoxygen to the bioreactor. Bubbles of from about 20 to about 50 mm indiameter, preferably from about 25, 30, or 35 to about 40 or 45 mm indiameter, are particularly effective in scouring the membranes. Bubblesof from about 3 to about 20 mm in diameter, preferably from about 4, 5,6, 7, 8, 9, or 10 mm to about 11, 12, 13, 14, 15, 16, 17, 18, or 19 mmin diameter, are generally preferred as providing both acceptableoxygenation and scouring.

Referring to FIG. 13, another embodiment of the invention is shown. Inthis embodiment the wastewater is fed through a conduit 59 into ananoxic reactor or reactors 60, which preferably utilize a mechanicalmixer (not shown) to keep solids in suspension without aeration. Theanoxic reactor 60 is fluidly connected to three further treatmentreactors 61, 62, 63 connected in series. Treatment reactor 63 is fluidlyconnected to a membrane filter 64 of the type described above. Activatedsludge collected by the membrane filter 64 is returned to the anoxicreactor 60 and further reactors 61 and 62 via return line 65. A portionof mixed liquor may be returned to anoxic zone 60 from treatment reactor62 through nitrate return line 66. Treatment reactor 63 is typicallyoperated as an anoxic zone with an input line 67 for selective carbondosing to assist nitrate removal. The carbon dosing may be controlled byproviding an online nitrate analyser 68 at the output from the membranefilter 64 to measure nitrate levels and feed back a signal to controlcarbon dosing to the second anoxic reactor 63.

The arrangement shown in this embodiment allows the dissolved oxygencontained in the liquid fed back from the membrane filter 64 to beselectively directed to the anoxic reactor 60 and used for simultaneousnitrification and denitrification of mixed liquor in the reactor 60 whenthe load to the treatment system is that required for operating atoptimal conditions. If the load is significantly less than the optimalload, the mass of dissolved oxygen in the return flow may exceed thatwhich can be consumed by bacteria in the anoxic reactor 60. For thiscondition, this embodiment provides the ability for the return flow tobe selectively directed to one or more of the further reactorsdownstream of the anoxic reactor 60 with nitrates being recirculatedfrom further reactor 62 to the anoxic reactor 60 to be denitrified afterat least some of the dissolved oxygen in the return flow from themembrane filter 64 has been consumed by bacteria in the downstreamfurther reactors 61 and 62.

In one form of this embodiment, the return flow from the membrane filter64 is directed to reactor 62 which is operated as an aerobic zone withreactor 61 operated as an aerobic zone. This allow some of the dissolvedoxygen in the return flow to removed by the bacteria in the mixed liquorcontained in reactor 62 before it is returned to the anoxic reactor 60.This arrangement ensures a stronger anoxic condition in anoxic reactor60 for promoting faster denitrification.

In another form of this embodiment, the return flow from the membranefilter 64 is directed to reactor 61 which is operated as an aerobic zonewith reactor 62 operated as an anoxic zone. This arrangement serves tofurther reduce the amount of dissolved oxygen recirculated to the anoxicreactor 60.

As can be seen from above a variety of different combinations of furtherreactors operating as different biological treatment zones may be usedwithout departing from the spirit and scope of the invention described.

Optionally, a mixing device may be employed in the anoxic reactor 60 toimprove mixing of the influent and the activated sludge. When theinfluent wastewater strength is low and/or the ratio of influent totalsuspended solids (TSS) to influent biochemical oxygen demand (BOD) ishigh, it may be difficult to maintain an oxygen deficit in the anoxiczone of the anoxic reactor 60 if an aeration device used for mixing. Forsuch applications, it may be desirable to use a mechanical mixer in theanoxic reactor 60. It will be appreciated that one or more of thedownstream reactors may still be operated as aerated-anoxic reactors torealize energy savings and faster removal of nitrogen. Aeration may beprovided intermittently during periods of high load.

All references cited herein are incorporated herein by reference intheir entirety, and are hereby made a part of this specification. To theextent publications and patents or patent applications incorporated byreference contradict the disclosure contained in the specification, thespecification is intended to supersede and/or take precedence over anysuch contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention as embodied in the attached claims.

1. A process for treating BOD, nitrogen and phosphorus containingwastewater, the process comprising the steps of: providing wastewaterinfluent into an anoxic zone having activated sludge; mixing thewastewater influent with the activated sludge in the anoxic zone to forma mixed liquor; providing the mixed liquor into one more furthertreatment zones; transferring a portion of the mixed liquor from thefurther treatment zone to a membrane filter wherein a filtrate isseparated from sludge; and selectively recycling at least a portion ofthe sludge to the anoxic zone as recycled activated sludge.
 2. Theprocess according to claim 1, wherein including the step of selectivelyrecycling at least a portion of the sludge to one or more of the furthertreatment zones.
 3. The process according to claim 1, wherein theportion of sludge recycled to the anoxic zone includes dissolved oxygen.4. The process according to claim 3 wherein the dissolved oxygen contentof the recycled flow is less than 8 mg/L.
 5. The process according toclaim 3 wherein the dissolved oxygen content of the recycled flow isabout 4 mg/L to about 8 mg/L.
 6. The process according to claim 3,wherein the dissolved oxygen is provided by the membrane filter.
 7. Theprocess according to claim 1, wherein in the anoxic zone promotessimultaneous nitrification and denitrification.
 8. The process accordingto claim 1, further comprising recycling a portion of mixed liquor fromthe further treatment zone to the anoxic zone for mixing therein withthe wastewater and the recycled activated sludge.
 9. The processaccording to claim 1 wherein one of said further treatment zones is afurther anoxic zone.
 10. A process according to claim 9 including thestep of providing a source of carbon to the further anoxic zone toassist denitrification.
 11. A process according to claim 10 includingthe step of measuring nitrate levels in the filtrate and controlling thesource of carbon provided to the further anoxic zone in dependence onthe measured nitrate levels.